To improve high temperature stability over amine cured epoxy resin systems and to give better physical and electrical properties, it has been the general practice in the epoxy technological field to use anhydride curing agents with glycidyl ether epoxy resins. Most anhydride formulations require elevated-temperature cures, and for most commercial applications, it is necessary to add some form of catalyst to speed the rate of cure. Consequently, a considerable amount of effort has been devoted in recent years to develop a perfect latent catalyst for curing glycidyl ether epoxy resins.
The properties desired of such a latent catalyst are the following:
A. It should give rapid cure of epoxy resins at moderately elevated temperatures (i.e., 135.degree. - 180.degree.C). PA1 b. It should be completely miscible with the resins at all temperatures. PA1 C. The storage life of the catalyzed resin should be indefinite. In practice, the viscosity of the resin should not change appreciably at room temperature after periods of 1 to several months. PA1 D. It should not adversely affect the properties of the cured resin. In particular, the electrical and mechanical properties of the resin should not be affected by the catalyst.
Several latent catalysts have appeared on the commercial scene in recent years. Included are quaternary ammonium halides such as benzyltrimethyl-ammonium chloride, stannous octoate, "extra-coordinate" siliconate salts, triethanolamine borate, triethanolamine titanate and various other metal chelates. However, all of these materials have been rejected for one reason or another and the quest for improved, perfect latent catalysts for anhydride cured glycidyl ether epoxy resins continues. Dante et al., U.S. Pat. No. 3,547,885, taught tetraphosphonium halide catalysts, but these proved to be of an ionic nature, subject to decomposition. Smith in U.S. Pat. No. 3,784,583, assigned to the assignee of this invention, taught quaternary organic phosphonium propionates, acetates, butyrates, isobutyrates and dimethyl phosphates as particularly effective latent catalysts.
It is believed that the prior art tetraphosphonium halides, like quaternary ammonium halides, decompose during cure to form organo-phosphine or amine compounds and halide salts. The inorganic salt fragments are not bound tightly to the epoxy resin and are free to react with moisture to form strong acids which could harm encapsulated electrical components. The prior art non-halide quaternary phosphonium slats are more thermally stable, due apparently to their more covalent bonding character, and appear to decompose only to a small extent on cure, forming organic rather than inorganic fragments. The need continues, however, for even more effective materials.